1. Field of the Invention
This invention relates to a data transmission system for transmitting binary information data.
2. Description of the Related Art
In transmitting a digital signal, one finds a pulse wave form having an extremely great amount of a low band frequency component if the input data consectively includes "1" or "0". If such digital data is, for example, to be recorded by a magnetic recording/reproducing apparatus such as a digital VTR, the low band frequency component would be cut off to make transmission difficult because of the use of a rotary transmitter. Then, it would increase the error rate of the data. Heretofore, in transmitting digital data, especially in the magnetic recording of a digital data signal or the like, practice has been to convert, for example, data of eight bits into data of nine bits (8/9 coding) for the purpose of suppressing the low band frequency component of the digital data signal to be recorded. In accordance with the above-stated 8/9 coding method, one redundant bit is added to every 8 bits. Therefore, while the low band component can be suppressed by that method, the redundancy of the signal increases.
Methods for avoiding the increase in redundancy include an n/n mapping coding method in which data of an n number of bits is converted into data of n bits. The n/n mapping coding method is applicable to a case where correlation is strong between adjacent data in terms of the statistical characteristic of the input signal. In accordance with this method, the low band frequency component of a coded data sequence is suppressed by utilizing this characteristic. More specifically, an incoming signal is difference coded. Differential data thus obtained shows a Laplace distribution converging in the neighborhood of zero of the positive and negative quantization levels. With this utilized, the low band component of a mapping coded data sequence is suppressed by converting the data sequence into a data sequence obtained with the DSV (digital sum value) of the data sequence selected as a parameter. This method includes, for example, a 4/4 mapping coding method in which input differential data of four bits is converted into data of four bits.
Further, with the level "1" of each bit in the bit pattern of data assumed to be "+1" and the level "0" to be "-1", the above-stated value DSV represents a total of bits within a single code. Accordingly, if the sum of bits of "1" is equal to that of the bits "0", the value DSV becomes zero. In this event, the data includes no DC component. The DC component of data increases according as the absolute value of DSV increases. In that event, the value DSV becomes a parameter showing the characteristic of the data.
While the low band frequency component of a coded data sequence thus can be suppressed without any increase in redundancy by mapping coding, the DC component cannot be completely removed by this method because of the n/n conversion ("n" represents a positive integer). Further, in case that some other additional information data, such as data for detecting and correcting error, is to be inserted in the coded data sequence, the low band suppressing effect attainable on the data sequence is greately lowered. As a result, the code error rate increases at the time of decoding.
FIG. 1 of the accompanying drawings shows an example of the arrangement of the conventional data frame. In FIG. 1, a part "information data" represents the abovestated mapping coded data sequence; "error detecting-correcting code" represents a check digit of, for example, a Hamming code, a Reed-Solomon code or the like.
With the data frame arranged as shown in FIG. 1, however, the low band suppressing effect on the data sequence greatly decreases in case that such checking information data as an error detecting-correcting code is inserted in the coded data sequence. Further, as a result of that, the code error rate increases at the time of demodulation.
FIG. 2 shows a case wherein a plurality of data frames each of which is arranged as shown in FIG. 1 and is provided with an inner code (row check code) are vertically aligned with an outer code (column check code) arranged in the vertical direction. The data frame arrangement thus, as a whole, forms a product code. Since this is a two-dimensional arrangement of the information data and the check codes, it is suitable particularly for picture data. However, in the case of the arrangement of FIG. 2, there is no correlation between data at each part where the check digits of inner and outer codes continue on each other. The lack of correlation does not permit use of a mapping code. Therefore, the suppressing effect on the low band frequency component decreases to a great degree. In the case of a data frame consisting solely of the check digits of outer and inner codes in particular, the check digits become consecutive over an excessively long period of time. Then, the low band frequency component suppressing effect for a portion of the data sequence at of such a long consecutive part decreases greatly.